1. Field of the Invention
The present invention relates to ink jet printing apparatus and more particularly to improved constructions, for the print head assembly of such apparatus, that eliminate malfunctions due to undesired liquid accumulations.
2. Description of the Prior Art
The term "continuous" has been used in the field of ink jet printer apparatus to characterize the types of ink jet printers that utilize continuous streams of ink droplets, e.g. in distinction to the "drop on demand" types. Continuous ink jet printers can be of the binary type (having "catch" and "print" trajectories for droplets of the continuous streams) and of the multi-deflection type (having a plurality of print trajectories for droplets of the continuous streams). Binary type apparatus most often employs a plurality of droplet streams while multi-deflection apparatus most often employs a single droplet stream.
In general, continuous ink jet printing apparatus have an ink cavity to which ink is supplied under pressure so as to issue in a stream from an orifice plate that is in liquid communication with the cavity. Periodic perturbations are imposed on the liquid stream (e.g vibrations by an electromechanical transducer) to cause the stream to break up into uniformly sized and shaped droplets. A charge plate is located proximate the stream break-off point to impart electrical charge in accord with a print information signal and charged droplets are deflected from their nominal trajectory. In one common binary printing apparatus charged droplets are deflected into a catcher assembly and non-charged droplets proceed along their nominal trajectory to the print medium.
The components described above (particularly the orifice plate and charge plate) must be precisely positioned to achieve proper charging, deflection and catching of non-print drops. Even after this is achieved, however, significant problems are presented at each operational start-up. For example, liquid accumulations on the charge plate can cause shorting or improper charging of droplets. Liquid accumulations at undesired locations on the catcher assembly can affect droplet deflection or impede droplet passage to the print medium.
Sophisticated prior art techniques have been developed to avoid the occurrence of such unwanted liquid accumulations on critical surfaces of the print head assembly. For example, complicated routines have been developed for starting-up and shutting down the ink jet streams without depositing ink on the critical print head surfaces. Also, air purging cycles have been provided to remove ink that is undesirably deposited on those surfaces. In some instances it is necessary to stop operation and physically clean the critical print head surfaces.
We have found that even when such procedures are meticulously practiced, malfunctions such as shorting of the drop charging electrodes and inconsistent droplet flights continue to appear at unacceptably short printing intervals. This problem was particularly severe with compact print head structures wherein the lower print head is located quite close to the ink jet printing streams.
After considerable investigation we discovered that one significant cause of such malfunctioning is the build-up of a clear liquid on the critical surfaces of the print head assembly. By further studies we discovered the cause of such clear liquid accumulation and structural embodiments which eliminate it, thus markedly reducing the frequency of ink jet printing apparatus malfunctions.